1. Field of the Invention
This invention relates to bipolar programmable read-only memories (often called "PROMs") and in particular to a process for programming junction-programmable PROMs.
2. Prior Art
Programmable read-only memories are well-known in the semiconductor arts. One of the first patents disclosing a programmable semiconductor memory is the Price U.S. Pat. No. 3,191,151 issued June 22, 1965 disclosing a programmable diode array. The Price array comprises a plurality of pairs of two diodes connected back-to-back. The array is programmed by sending a current pulse of the proper magnitude and direction through selected ones of the back-to-back diodes thereby converting the junction of each of these back-biased diodes to a short circuit without affecting the connected forward-biased diode. Price also discloses the use of a fuse which when destroyed converts the circuit from conducting to non-conducting. Additional patents disclosing programmable read-only memories include U.S. Pat. Nos. 3,733,690; 3,742,592; and 3,848,238. These patents disclose both a programmable read-only memory using avalanche breakdown to program selected devices and a method for achieving avalanche junction breakdown much as in the Price U.S. Patent No. 3,191,151.
Heretofore it has not been feasible to combine the avalanche breakdown of a PN junction with the recessed oxide isolation structure of the prior art (such as disclosed by Peltzer in U.S. Pat. No. 3,648,125 issued Mar. 7, 1972) to yield a programmable read only memory programmable by avalanche breakdown of a PN junction because the voltages required to break down such a junction were close to the breakdown voltages of oxide isolated structures. However, Shideler and Mishra, in U.S. patent application Ser. No. 336,802 filed Jan. 4, 1982, entitled "Oxide Isolation Process for Standard RAM/PROM and Lateral PNP Cell RAM" and assigned to Fairchild Camera and Instrument Corporation, the assignee of this application, turn to advantage certain of the features of the prior art oxide isolation process to provide a structure which is particularly useful in the manufacture of junction-programmable read-only memories (PROMs). In accordance with Shideler's and Mishra's invention, the breakdown voltages associated with prior art recessed oxide isolation structures and particularly the sidewall regions of the isolated semiconductor islands adjacent the recessed oxide are substantially increased. This is done by using an N type epitaxial layer on a P type substrate and turning to advantage the pile up of N type impurities adjacent the recessed oxide resulting from thermal oxidation of the epitaxial layer and by preventing P type impurities used to form channel stops in the field of the device from reaching the sidewalls of the silicon islands. In addition, the collector-emitter breakdown voltage (LV.sub.CEO) of NPN vertical transistors formed in the semiconductor islands when this process is used to make PROMs is increased by implanting a P type impurity in those portions of the silicon islands to be occupied by the P type base regions of the NPN vertical transistors.
In one embodiment of the Shideler-Mishra invention, a PROM is made from a plurality of transistors each having an emitter, a floating base and a collector, wherein the emitter-base junction of each of selected transistors is destroyed by a programming current, thereby eliminating the need for prior art fuses (such as nichrome fuses) and the extra processing and yield difficulties associated with the nichrome process. The Shidelar-Mishra invention also allows the use of fuses for "redundant" type RAMs.
The fuses for use in a PROM device in accordance with the Shidelar-Mishra invention are walled emitter transistors (that is, transistors the emitter regions of which are laterally defined by the recessed isolation or "field" oxide) which are avalanched so as to break down the emitter-base junction.
One of the disadvantages of the prior art techniques for programming junctions is that the yield of the programmed read-only memories is lower than desired. Among the reasons for this yield loss is damage to the base-collector junctions of programmed cells due to excessive current passing through the cells after avalanching of the emitter-base junctions. Using current pulses of a constant width to program a cell by destroying the base-emitter junction often results in excessive current through the device with commensurate damage to the base-collector junction.